This specification provides a way for an author to specify, in CSS, the size, zoom factor, and orientation of the viewport that is used as the base for the initial containing block.
This section is not normative.
CSS 2.1 [[!CSS21]] specifies an initial containing block for continuous media that has the dimensions of the viewport. Mobile/handheld device browsers have a viewport that is generally a lot narrower than a desktop browser window at a zoom level that gives a CSS pixel size recommended by CSS 2.1.
The narrow viewport is a problem for documents designed to look good in desktop browsers. The result is that mobile browser vendors use a fixed initial containing block size that is different from the viewport size, and close to that of a typical desktop browser window. In addition to scrolling or panning, zooming is often used to change between an overview of the document and zoom in on particular areas of the document to read and interact with.
Certain DOCTYPEs (for instance XHTML Mobile Profile) are used to recognize mobile documents which are assumed to be designed for handheld devices, hence using the viewport size as the initial containing block size.
Additionally, an HTML META tag has been introduced for allowing an
author to specify the size of the initial containing block, and the initial
zoom factor directly. It was first implemented by Apple for the Safari/iPhone
browser, but has since been implemented for the Opera, Android, and Fennec
browsers. These implementations are not fully interoperable and this
specification is an attempt at standardizing the functionality provided by
the viewport META tag in CSS.
This specification follows the CSS property definition conventions from [[!CSS3SYN]].
Value types are defined in [[!CSS3VAL]].
This specification introduces a way of overriding the size of the viewport provided by the user agent (UA). Because of this, we need to introduce the difference between the initial viewport and the actual viewport.
When the actual viewport cannot fit inside the window or viewing area, either because the actual viewport is larger than the initial viewport or the zoom factor causes only parts of the actual viewport to be visible, the UA should offer a scrolling or panning mechanism.
It is recommended that initially the upper-left corners of the
actual viewport and the window or viewing area are aligned if the
base direction of the document is ltr. Similarly, that the upper-right
corners are aligned when the base direction is rtl. The base direction for
a document is defined as the computed value of the
direction property for the first
BODY element of an HTML or XHTML document. For
other document types, it is the computed
direction for the root element.
@viewport ruleThe @viewport at-rule
consists of the @-keyword followed by a block of property
declarations describing the viewport.
The property declarations inside an @viewport
rule are per document properties and there is no inheritance involved. Hence
declarations using the ‘inherit’
keyword will be dropped. They work similar to @page
properties and follow the cascading order of CSS. Hence, properties in
@viewport rules will override properties from
preceding rules. The declarations allow !important which will affect
cascading of properties accordingly.
Should the @viewport rule apply to top-level documents only? If not, we need to say something about different zoom factors in frames.
This example sets the viewport to fit the width of the device. Note that it is enough to set the width as the height will be resolved from the width.
@viewport {
width: device-width;
}
The syntax for the @viewport rule is as follows
(using the notation from the Grammar appendix of CSS
2.1 [[!CSS21]]):
viewport
: VIEWPORT_SYM S*
'{' S* declaration? [ ';' S* declaration? ]* '}' S*;
with the new token:
@{V}{I}{E}{W}{P}{O}{R}{T} {return VIEWPORT_SYM;}
where:
V v|\\0{0,4}(56|76)(\r\n|[ \t\r\n\f])?|\\v
W w|\\0{0,4}(57|77)(\r\n|[ \t\r\n\f])?|\\w
The viewport non-terminal is added to the stylesheet
production along with the ruleset, media, and
page non-terminals:
stylesheet
: [ CHARSET_SYM STRING ';' ]?
[S|CDO|CDC]* [ import [ CDO S* | CDC S* ]* ]*
[ [ ruleset | media | page | viewport ] [ CDO S* | CDC S* ]* ]*
;
It is also added to media production to allow
@viewport rules nested inside
@media rules This is extending
the CSS 2.1 syntax. A draft of CSS3 Paged Media also allows page inside
@media.:
media : MEDIA_SYM S* media_list LBRACE S* [ ruleset | viewport ]* '}' S* ;
This section presents the properties that are allowed inside an
@viewport rule. Other properties than those
listed here will be dropped.
Relative length values are resolved against initial values. For instance 'em's are resolved against the initial value of the font-size property.
min-width’ and
‘max-width’ properties| Name: | min-width |
| Value: | <viewport-length> |
| Initial: | auto |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | Refer to the width of the initial viewport |
| Media: | visual, continuous |
| Computed value: | ‘auto’,
‘device-width’,
‘device-height’, an
absolute length, or a percentage as specified |
| Name: | max-width |
| Value: | <viewport-length> |
| Initial: | auto |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | Refer to the width of the initial viewport |
| Media: | visual, continuous |
| Computed value: | ‘auto’,
‘device-width’,
‘device-height’, an
absolute length, or a percentage as specified |
Specifies the minimum and maximum width of the viewport that is used to set the size of the initial containing block where
<viewport-length> = auto | device-width | device-height | <length> | <percentage>
and the values have the following meanings:
auto’device-width’device-height’A positive absolute or relative length.
A percentage value relative to the width or height of the initial viewport at zoom factor 1.0, for horizontal and vertical lengths respectively. Must be positive.
The min-width and max-width properties are inputs to the constraining procedure. The width will initially be set as close as possible to the initial viewport width within the min/max constraints.
width’
shorthand property| Name: | width |
| Value: | <viewport-length>{1,2} |
| Initial: | See individual properties |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | See individual properties |
| Media: | visual, continuous |
| Computed value: | See individual properties |
This is a shorthand property for setting both min-width and max-width. One <viewport-length> value will set both min-width and max-width to that value. Two <viewport-length> values will set min-width to the first and max-width to the second.
min-height’ and
‘max-height’ properties| Name: | min-height |
| Value: | <viewport-length> |
| Initial: | auto |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | Refer to the height of the initial viewport |
| Media: | visual, continuous |
| Computed value: | ‘auto’,
‘device-width’,
‘device-height’, an
absolute length, or a percentage as specified |
| Name: | max-height |
| Value: | <viewport-length> |
| Initial: | auto |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | Refer to the height of the initial viewport |
| Media: | visual, continuous |
| Computed value: | ‘auto’,
‘device-width’,
‘device-height’, an
absolute length, or a percentage as specified |
Specifies the minimum and maximum height of the viewport that is used to set the size of the initial containing block.
The min-height and max-height properties are inputs to the constraining procedure. The height will initially be set as close as possible to the initial viewport height within the min/max constraints.
height’ shorthand property| Name: | height |
| Value: | <viewport-length>{1,2} |
| Initial: | See individual properties |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | See individual properties |
| Media: | visual, continuous |
| Computed value: | See individual properties |
This is a shorthand property for setting both min-height and max-height. One <viewport-length> value will set both min-height and max-height to that value. Two <viewport-length> values will set min-height to the first and max-height to the second.
zoom’ property| Name: | zoom |
| Value: | auto | <number> | <percentage> |
| Initial: | auto |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | The zoom factor itself |
| Media: | visual, continuous |
| Computed value: | ‘auto’, or a positive
number or percentage as specified. |
Specifies the initial zoom factor for the window or viewing area. This is a magnifying glass type of zoom. Interactively changing the zoom factor from the initial zoom factor does not affect the size of the initial or the actual viewport.
Values have the following meanings: Should both numbers and percentages be allowed?
auto’auto’ values
for ‘zoom’.A positive number used as a zoom factor. A factor of 1.0 means that no zooming is done. Values larger than 1.0 gives a zoomed-in effect and values smaller than 1.0 a zoomed-out effect.
A positive percentage value used as a zoom factor. A factor of 100% means that no zooming is done. Values larger than 100% gives a zoomed-in effect and values smaller than 100% a zoomed-out effect.
min-zoom’ property| Name: | min-zoom |
| Value: | auto | <number> | <percentage> |
| Initial: | auto |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | The zoom factor itself |
| Media: | visual, continuous |
| Computed value: | ‘auto’, or a positive
number or percentage as specified. |
Specifies the smallest allowed zoom factor. It is used as input to the
constraining procedure to constrain
non-‘auto’
‘zoom’
values, but also to limit the allowed zoom factor that can be set through
user interaction. The UA should also use this value as a constraint when
choosing an actual zoom factor when the used value of
‘zoom’ is
‘auto’.
Values have the following meanings:
auto’zoom’ property used in the
constraining procedureA positive number limiting the minimum value of the zoom factor.
A positive percentage limiting the minimum value of the zoom factor.
max-zoom’ property| Name: | max-zoom |
| Value: | auto | <number> | <percentage> |
| Initial: | auto |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | The zoom factor itself |
| Media: | visual, continuous |
| Computed value: | ‘auto’, or a positive
number or percentage as specified. |
Specifies the largest allowed zoom factor. It is used as input to the
constraining procedure to constrain
non-‘auto’
‘zoom’
values, but also to limit the allowed zoom factor that can be set through
user interaction. The UA should also use this value as a constraint when
choosing an actual zoom factor when the used value of
‘zoom’ is
‘auto’.
Values have the following meanings:
auto’zoom’
property used in the constraining
procedureA positive number limiting the maximum value of the zoom factor.
A positive percentage limiting the maximum value of the zoom factor.
user-zoom’ property| Name: | user-zoom |
| Value: | zoom | fixed |
| Initial: | zoom |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | N/A |
| Media: | visual, continuous |
| Computed value: | ‘zoom’ or
‘fixed’ as specified. |
Specifies if the zoom factor can be changed by user interaction or not.
Values have the following meanings:
zoom’fixed’orientation’ property| Name: | orientation |
| Value: | auto | portrait | landscape |
| Initial: | auto |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | N/A |
| Media: | visual, continuous |
| Computed value: | ‘auto’,
‘portrait’, or
‘landscape’ as specified. |
This property is used to request that a document is displayed in portrait or landscape mode. For a UA/device where the orientation is changed upon tilting the device, an author can use this property to inhibit the orientation change.
Values have the following meanings:
auto’portrait’landscape’resolution’ propertyThis property is at risk.
| Name: | resolution |
| Value: | auto | device | <resolution> |
| Initial: | auto |
| Applies to: | N/A |
| Inherited: | N/A |
| Percentages: | N/A |
| Media: | visual, continuous |
| Computed value: | ‘auto’,
‘device’,
or a resolution value as specified. |
The UA relates the CSS pixel to the reference pixel or the physical length units as described in CSS 2.1 [[!CSS21]]. The resolution property can be used to override the CSS pixel size chosen by the UA by setting a CSS pixel resolution. In CSS, absolute length units are fixed in relation to each other, hence, changing the CSS pixel resolution will for instance change the physical length of a CSS cm.
The relationship between a device pixel and a CSS pixel will depend on
the device resolution. Setting resolution: 120dpi on a 240dpi
device will give a 2:1 ratio between device and CSS pixels, while on a
120dpi device, the ratio will be 1:1. Likewise, the
‘device’ value will always keep a
1:1 ratio, but will give different CSS pixel resolution depending on
device resolution.
Note that setting this property will affect the used lengths for
‘device-width’,
‘device-height’, and the size of the
initial viewport.
The <resolution> value is a positive <number> immediately
followed by a unit identifier (‘dpi’
or ‘dpcm’).
Values have the following meanings:
auto’device’For the procedure below:
Properties refer to the values resolved/constrained to at that point in the procedure. They are initially resolved to their computed values.
width and
height refer to the
resolved viewport size and not the shorthand properties. They are both
initially ‘auto’.
MIN/MAX computations where one of the arguments is
‘auto’ resolve to the other argument.
For instance, MIN(0.25, 'auto') = 0.25, and
MAX(5, 'auto') = 5.
initial-width is the
width of the initial viewport in pixels at zoom factor 1.0.
On a device this is typically (device-width
- decorations). On Safari/iPhone this is identical to the
device-width.
initial-height is the
height of the initial viewport in pixels at zoom factor 1.0.
Typically (device-height - decorations).
The used values are resolved from the computed values going through the steps below. The procedure is closely modelled after the behavior of the Safari/iPhone browser. We have run tests on the Android SDK browser and a Fennec nightly build for Windows to see where the various implementations are the same and where they differ. We found that the Safari implementation is by far the most consistent and predictable, and decided to ignore the other implementations for this specification. This procedure, and the parsing algorithm, are backed up by a compliance test suite currently consisting of 120-130 tests. All but one pass in the tested Safari browser.
User agents are expected, but not required, to re-run this procedure and re-layout the document, if necessary, in response to changes in the user environment, for example if the device is tilted from landscape to portrait mode or the window that forms the initial viewport is resized.
auto’ lengths to pixel lengthsdevice-width’,
‘device-height’) to pixel
values for the ‘min-width’,
‘max-width’,
‘min-height’ and
‘max-height’
propertieswidth and
height from min/max propertiesmin-width or
max-width is not
‘auto’, set
width = MAX(min-width, MIN(max-width, initial-width))min-height or
max-height is not
‘auto’, set
height = MAX(min-height, MIN(max-height, initial-height))min-zoom
and max-zoom valuesmin-zoom is not
‘auto’ and
max-zoom is not
‘auto’,
set max-zoom = MAX(min-zoom, max-zoom)zoom
value to the [min-zoom, max-zoom] rangezoom is not
‘auto’,
set zoom = MAX(min-zoom, MIN(max-zoom, zoom))width
valuewidth
and zoom are both
‘auto’, set width =
initial-widthwidth is
‘auto’, and
height is
‘auto’, set width =
(initial-width / zoom)width is
‘auto’, set width = height *
(initial-width / initial-height)height
valueheight is
‘auto’, set height = width *
(initial-height / initial-width)width
and height to fill the
window/viewing area for the specified zoomzoom
or max-zoom is not
‘auto’, set width =
MAX(width, (initial-width / MIN(zoom, max-zoom)))zoom
or max-zoom is not
‘auto’, set height =
MAX(height, (initial-height / MIN(zoom, max-zoom)))
This example shows the case where the used value for width is increased to fit the window/viewing area for a specified zoom value. The used value for width will be two times device-width in this case, assuming device-width is the same as the initial viewport width.
@viewport {
width: device-width;
zoom: 0.5;
}
For several media features, the size of the initial containing block and
the orientation of the device affects the result of a media query
evaluation, which means that the effect of
@viewport rules on media queries needs extra
attention.
From the Media Queries specification [[!MEDIAQ]]:
“To avoid circular dependencies, it is never necessary to apply the style sheet in order to evaluate expressions. For example, the aspect ratio of a printed document may be influenced by a style sheet, but expressions involving ‘device-aspect-ratio’ will be based on the default aspect ratio of the user agent.”For
@viewport rules, though, it is recommended
that they are applied before media queries for other rules are evaluated.
Recommended procedure for applying CSS rules:
@viewport rules. If
@viewport rules rely on media queries, use
the viewport properties of the initial viewport.The rationale for using the viewport properties obtained from applying
the @viewport rules for evaluating media
queries for style rules, is that media queries should match the
actual viewport that the document will be
layed out in and not the initial or the one specified in the UA stylesheet.
Consider the example below given that the UA stylesheet has a viewport
width of 980px, but a device-width and initial
viewport width of 320px. The author has made separate styles to make
the document look good for initial containing block widths above or below
400px. The actual viewport used will be
320px wide, and in order to match the styles with the
actual viewport width, the viewport
resulting from applying the @viewport rules
should be used to evaluate the media queries.
Given a device-width of 320px and a UA stylesheet viewport width of 980px, the first media query will not match, but the second will.
@viewport {
width: device-width;
}
@media screen and (min-width: 400px) {
div { color: red; }
}
@media screen and (max-width: 400px) {
div { color: green; }
}
Another example:
The media query below should match because the
@viewport rule is applied before the media
query is evaluated.
@media screen and (width: 397px) {
div { color: green; }
}
@viewport {
width: 397px;
}
Below is an example where an @viewport rule
relies on a media query affected by the viewport properties.
The green color should be applied to a div because the
initial viewport
width is used to evaluate the media query for the second
@viewport rule, but the
actual viewport is
used for evaluating the media query when applying style rules.
@viewport {
width: 397px;
}
@media screen and (width: 397px) {
@viewport {
width: 500px;
}
}
@media screen and (width: 397px) {
div { color: green; }
}
It is recommended that authors do not write
@viewport rules that rely on media queries
whose evaluation is affected by viewport properties. Is is also
recommended that the @viewport rule(s) is
placed as early in the document as possible to avoid unnecessary
re-evaluation of media queries or reflows.
The next example illustrates possible circular dependencies between media
queries and @viewport rules. Assuming a UA
stylesheet viewport width larger than 200px, the first viewport rule would
apply causing an actual viewport width of
100px. If the media queries were based on the
actual viewport, a re-evaluation would
apply the second @viewport rule which would
in turn cause the first media query to be true, which means we're back to
start.
@media screen and (min-width: 200px) {
@viewport {
width: 100px;
}
}
@media screen and (max-width: 200px) {
@viewport {
width: 300px;
}
}
There are prefixed implementations of a media feature for the device:css pixel ratio (-webkit-device-pixel-ratio / -o-device-pixel-ratio). Should be standardized here or in a new level of Media Queries?
Properties in the CSSOM and CSSOM View specifications refer to the viewport and the initial containing block. If any of those properties should refer to the initial viewport and not the actual viewport, those exceptions need to be adressed.
Standardize window.devicePixelRatio? Should be done in the CSSOM View spec perhaps?
Requirements for a conforming UA:
The ‘min-width’,
‘max-width’,
‘width’,
‘min-height’,
‘max-height’, and
‘height’ properties must be supported.
The ‘min-zoom’,
‘max-zoom’, and
‘zoom’ properties must be supported as
input to the
constraining procedure. However,
the UA may choose to use a different zoom factor when presenting the
document to the user, and use different minimum and maximum zoom
limits for the user interaction.
This will for instance allow UAs without zooming capabilities to conform and still have interoperable implementations when it comes to viewport dimensions. It will also allow the UA to choose a different zoom factor when content overflows the actual viewport.
Support for the ‘user-zoom’
and ‘orientation’
properties is optional.
Need to say something about the resolution property if it is kept.
META elementThis section is not normative.
This section describes a mapping from the content attribute of the
viewport META element, first implemented by Apple in the iPhone
Safari browser, to the properties of the
@viewport rule described in this
specification.
In order to match the Safari implementation, the following parsing algorithm and translation rules rely on the UA stylesheet below.
@viewport {
width: 980px;
min-zoom: 0.25;
max-zoom: 5;
}
Note that these values might not fit well with all UAs. For instance, with a min-zoom of 0.25 you will be able to fit the whole width of the document inside the window for widths up to 1280px on a 320px wide device like the iPhone, but only 960px if you have 240px display.
The recognized properties in the viewport META element are:
widthheightinitial-scaleminimum-scalemaximum-scaleuser-scalabletarget-densityDpiAt
risk since ‘resolution’ is at
risk.Below is an algorithm for parsing
the content attribute of the META tag
produced from testing Safari on the iPhone. The testing was done on an iPod touch running iPhone OS 4.
The UA string of the browser: "Mozilla/5.0 (iPod; U; CPU
iPhone OS 4_0 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko)
Version/4.0.5 Mobile/8A293 Safari/6531.22.7". The pseudo
code notation used is based on the notation used in [Algorithms]. The white-space
class contains the following characters (ascii):
Parse-Content(S) i ← 1 while i ≤ length[S] do while i ≤ length[S] and S[i] in [white-space, ',', '='] do i ← i + 1 if i ≤ length[S] then i ← Parse-Property(S, i) Parse-Property(S, i) start ← i while i ≤ length[S] and S[i] not in [white-space, ',', '='] do i ← i + 1 if i > length[S] or S[i] = ',' then return i property-name ← S[start .. (i - 1)] while i ≤ length[S] and S[i] not in [',', '='] do i ← i + 1 if i > length[S] or S[i] = ',' then return i while i ≤ length[S] and S[i] in [white-space, '='] do i ← i + 1 if i > length[S] or S[i] = ',' then return i start ← i while i ≤ length[S] and S[i] not in [white-space, ',', '='] do i ← i + 1 property-value ← S[start .. (i - 1)] Set-Property(property-name, property-value) return i
Set-Property matches the
listed property names case-insensitively.
The property-value strings are interpreted
as follows:
property-value can be
converted to a number using strtod, the
value will be that number. The remainder of the string is
ignored.property-value string
will be matched with the following strings
case-insensitively: yes,
no, device-width,
device-height@viewport propertiesThe Viewport META element is placed in the
cascade as if it was a STYLE element, in the
exact same place in the dom, that only contains a single
@viewport rule.
Each of the property/value pair from the parsing in the previous section are translated, and added to that single at-rule as follows:
Unknown properties are dropped.
width
and height
propertiesThe width and
height viewport
META properties are translated into
‘width’ and
‘height’ shorthand properties,
effectively setting the min and max properties to the same value.
[1px, 10000px]device-width
and device-height
are used as keywordsFor a viewport META element that translates
into an @viewport rule with a
non-‘auto’
‘zoom’
declaration and no ‘width’
declaration, add:
width: auto;
to the @viewport rule.
This META element:
<meta name="viewport" content="initial-scale=1.0">
translates into:
@viewport {
zoom: 1.0;
width: auto;
}
initial-scale,
minimum-scale, and
maximum-scale propertiesThe properties are translated into
‘zoom’,
‘min-zoom’,
and ‘max-zoom’
respectively with the following translations of values.
[0.1, 10]yes is translated to 1device-width
and device-height
are translated to 10no and unknown values are translated to 0.1For a viewport META element that translates into an
@viewport rule with no ‘max-zoom’ declaration and a non-‘auto’
‘min-zoom’ value that is larger than the ‘max-zoom’ value of the UA stylesheet,
the ‘min-zoom’ declaration value is clamped to the UA stylesheet ‘max-zoom’
value.
user-scalable propertyThe user-scalable property is translated into
‘user-zoom’
with the following value translations.
yes and no are
translated into
‘zoom’ and
‘fixed’ respectively.device-width
and device-height
are mapped to
‘zoom’<-1, 1>, and unknown values,
are mapped to ‘fixed’This META element:
<meta name="viewport" content="width=480, initial-scale=2.0, user-scalable=1">
will translate into this @viewport block:
@viewport {
width: 480px;
zoom: 2.0;
user-zoom: zoom;
}
target-densityDpi propertyAt risk since ‘resolution’ is at risk.
This property differ from the others since it is from the WebKit implementation used in the Android browser and not supported in the Safari
The target-densityDpi property is translated
into ‘resolution’ with the
following value translations.
[70, 400] are translated to
‘dpi’ values.device-dpi translates to
‘device’low-dpi translates to 120dpimedium-dpi translates to 160dpihigh-dpi translates to 240dpiauto’
for ‘zoom’This section is not normative.
This section presents one way of picking an actual value for the
‘zoom’
property when the used value is
‘auto’.
Given an initial viewport with size (initial-width,
initial-height), and a finite region within the
canvas where
the formatting structure is rendered (rendered-width,
rendered-height). That region is at least as large as the
actual viewport.
Then, if the used value of
‘zoom’ is
‘auto’, let the
actual zoom factor be:
zoom = MAX(initial-width / rendered-width, initial-height / rendered-height)
The actual zoom factor should also be further limited by the [min-zoom, max-zoom] range.